The present invention relates to a solar cell panel comprising an array of interconnected solar cells within a space defined by a support member and a transparent cover member.
Physically, a solar cell array panel comprises an insulating substrate or support member, in the prior art usually a fiber-reinforced polyimide sheet such as Kapton.RTM. foil reinforced through bonding to a fibrous weave, a plurality of solar cells with inter-connectors bonded to the substrate and with a thin sheet of a clear, radiation-protection material such as a sheet of glass or fused silica bonded to the front of each of the solar cells.
A manufacturing process would include, inter alia, bonding the fiber reinforcement to the substrate through an adhesive, bonding the solar cells to the substrate laminate through the use of an adhesive and also bonding the quartz or glass covers by means of adhesive to the individual solar cells. The aforementioned manufacturing steps are in addition to interconnector welding, cutting of cover glasses, etc., which in combination with the aforementioned bonding steps have prevented the production of low-cost, light-weight, acceptable solar cell array panels.
Kapton.RTM. polyimide is available only in sheet form. It is difficult to reproducibly manufacture and inspect the bond between the reinforcement fibers, such as a graphite weave, and the Kapton.RTM. foil to provide the needed thermal and mechanical properties at minimum weight.
Recently, polyimide compositions have been developed which can be coated in liquid form onto the front surfaces of solar cells (i.e. the surfaces exposed to the external environment in the final solar cell assembly) to provide protection sufficient for use in outerspace. The limitations of other polymers, such as being colored, discoloring excessively, inadequate stability and the like are not found in the use of these new polyimide compositions, which also enable one to avoid the excessive weight of glass and quartz coverplates.
The above-referenced polyimide compositions useful to encapsulate solar cells are disclosed in co-pending application Ser. No. 451,137, filed Dec. 20, 1982, by DuPont and Bilow, expressly incorporated herein by reference. The DuPont and Bilow polyimide provides a coating which (1) is colorless, (2) is transparent to the solar radiation in the visible light spectrum, (3) is relatively non-brittle, (4) has a high degree of thermal stability, (5) readily transmits solar radiation without appreciable degradation, (6) is heat resistant, (7) does not degrade significantly when exposed to ultraviolet radiation, and (8) is highly effective in protecting against electrons and low energy proton radiation.
The above-described DuPont and Bilow polyimide coatings are formed from a polyimide composition which has the recurring structural unit shown below: ##STR1## where R is: ##STR2## and n has a value range from 10 to about 2000. Preferably, n has a value from about 20 to about 1000.
The polyimide which is most preferred according to the DuPont and Bilow invention is the meta amino phenylene derivative of formula II above, and having the recurring structural unit: ##STR3## and its precursor has the polyamic acid structure: ##STR4## where n has the values noted above.
The polyimide of formula IV above is prepared by the reaction of substantially equal molar proportions of the two monomers 2,2-bis(3-aminophenyl)hexafluoropropane and 4,4'-hexafluoroisopropylidene[bis(phthalic anhydride)], in a solvent for such monomers. The solvents which can be used include, for example, tetrahydrofuran, N-methyl pyrrolidinone, N-methylformamide, dimethylformamide and N,N-dimethylacetamide and mixtures thereof. The resulting polyamic acid solution can be cast as a film and the film imidized to the polyimide structure IV above. Both the polyamic acid and the polyimide have an inherent viscosity of at least 0.1, usually 0.3-0.5. The inherent viscosity of the polyimide is measured at 30.degree. C. as a 0.5% solution in a suitable solvent, such as cold concentrated (95%) sulfuric acid or methanesulfonic acid.
As noted above, in preparing the coated solar cells, a solution of the polyamic acid precursor of formula V above in a solvent, such as tetrahydrofuran, at a concentration of about 10 to about 30% of the polyamic acid, can be used as a varnish for application to the active surface of a solar cell.
The varnish or solution of the polyamic acid precursor can be coated over, where desired, a primer coating such as a silane adhesion promoter, or the solar cell in any suitable manner, for example, by dipping, electrocoating, spraying, electrostatic spraying and the like. A 15% solid content solution of the polymer in N-methylpyrrolidinone or dimethylformamide has been found to be effective. The solution is sprayed after applying the primer (Union Carbide A1100, aminopropyltrimethoxysilane) from a 5% solution in ethanol. The amount of the polyamic acid in the solvent will vary depending primarily on the type of sprayer or other coating means which is used. The solid content of the polyamic acid in the solvent solution can vary greatly and could be as high as 30% in tetrahydrofuran and in which solution dimethylformamide can be present in an amount of normally at least 60%.
After application of the polyamic acid varnish to the solar cell, that is, over the primer, the solvent is essentially evaporated off and the amic acid polymer is converted into the imidized or polyimide structure of formula IV by heating such amic acid polymer at about 250.degree. C. Lower temperatures, such as at 120.degree. C., can also be used to promote the imidization, but the reaction rate is slower and the elimination of solvent residues is slower. Preferred imidization temperatures range between about 160.degree. C. and 250.degree. C. Thinner coatings (about 0.1 mil thick) can be dried and then cured for 1 to 2 hr. at 485.degree. F. (about 250.degree. C.) in vacuum. However, the preferred temperature for effecting imidization is that which provides the best solar cell performance, and this may vary depending upon the specific type of cell and the specific batch of amic acid polymer available.
The polyimide film thus formed is generally a very thin layer, as is the silane primer film. The polyimide film itself is preferably about 0.2 to 0.5 mil thick. However, the practical minimum thickness is about 0.1 mil. There is no absolute maximum thickness, except that the film should be as thin as possible and yet provide the desired characteristics to protect the solar cell from radiation.
Although the employment of the above-described polyimide coatings has provided lighter weight solar cell array panels, the deficiencies existent in the Kapton.RTM.-type substrate and the need for at least two adhesive bonding steps in the manufacturing process remain.